Article and method for finding a compact representation to visualize complex decision trees

ABSTRACT

The invention comprises an article and method for transforming a complex or large decision tree having multiple variables; multiple values for each variable; and, multiple outcomes for each combination of variables and their associated values, into a compact, efficient graphical representation to provided enhanced ease of use and interaction by a human user. More particularly, the invention comprises a computationally efficient method for transforming an input decision tree into an optimal compact representation by computing a particular ordering of variables in the decision tree that first leads to a Directed Acyclic Graph, or “DAG,” with a minimum number of nodes. The method then converts the DAG into an exception-based DAG, or “EDAG,” with exactly one exception, having an optimal, minimum number of nodes with increased comprehensibility for a user.

CROSS-REFERENCES

The present application claims priority to related provisional patent application Ser. No. 60/823,618 filed Aug. 25, 2006, entitled “Strategy and Ruleset Visualization,” by inventors Stuart Crawford, Gaurav Chhaparwal, Kashyap KBR, Navin Doshi and Sergei Tolmanov, which is not admitted to be prior art with respect to the present invention by its mention in the background. This application is incorporated herein in its entirety by this reference thereto.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to the organization and viewing of information. More particularly, the invention relates to a methodology for efficiently transforming large or complex decision trees into compact, optimized representations to ease viewing and interaction by a user.

2. Discussion of the Related Art

A decision tree is a structure composed of nodes and links in which the end nodes, also called leaf nodes, represent actions to be taken, the interior nodes represent conditions to be tested on variables, and the branches represent conjunctions of conditions that lead to actions.

A decision tree can represent a series of decisions that divide a population into subsets. The first decision divides the population into two or more segments (i.e. partitions). For each of these segments, a second decision divides the segment into smaller segments. The second decision depends on the choice for the first decision, and the third decision depends on the choice for the second decision, and so on. In other words, a decision tree can be used to segment a dataset in such a way that an action node of the tree corresponds to that segment of the dataset for which all the conditions along the branch from root to that action node are satisfied.

A leveled decision tree is one where the variables appearing along each branch are always in the same order. A decision tree is said to be ‘read once’ when no variable appears twice along any branch.

Depending on the decision process being modeled, a decision tree can be extremely complex, having many variables, many values for those variables, and many outcomes depending on the various combinations of the variables and their associated values.

A sample leveled decision tree is shown in FIG. 1. The tree determines the type of credit card that a bank gives to its credit card applicants on the basis of their job, income and value of assets. In this example, “Job,” “Income” and “Assets” are the variables on which conditions are defined. For example, if a customer satisfies the conditions, “Job=Business AND Income<$100K” AND “Assets>$200K,” the conclusion leaf node reached is “Card=Bronze.”

Information presented in the form of a decision tree becomes difficult to comprehend and visualize when the tree is large. This invention relates to a method for finding an optimal ordering of the variables of the decision tree, and using that ordering to convert the tree to an optimal Directed Acyclic Graph, i.e., a “DAG,” such that the DAG represents the same information found in the tree but with the smallest possible number of nodes compared to any other ordering of variables.

A DAG is a Directed Graph with no cycles or loops. A Directed Graph is a set of nodes and a set of directed edges, also known as arcs or links, connecting the nodes. The edges have arrows indicating directionality of the edge.

Tree representations are comprehensible ‘knowledge structures’ when they are small, but become more and more incomprehensible as they grow in size. Comprehensibility of the knowledge structure is a critical issue since, ultimately, humans must work with and maintain such structures.

The reason why trees often become large is the repeated occurrence of identical subsets of conditions interspersed throughout the tree. This phenomenon is called “sub-tree replication.”

Others have attempted to resolve the problems associated with sub-tree replication. Ron Kohavi, in his research paper, “Bottom-up Induction of Oblivious Read-Once Decision Graphs,” published in the “European Conference on Machine Learning,” 1994, introduced a new representation for a decision tree, called the “Oblivious read-once Decision Graph,” i.e., the “OODG.” He also described a method to convert a tree representation to the OODG representation. However, Kohavi's method chooses the ordering of constituents in an ad hoc manner, which fails to ensure the resulting representation will have the least number of nodes.

Brian R. Gaines, in his research paper “Transforming Rules and Trees into Comprehensible Knowledge Structures,” suggests an alternative representation for general decision trees called the “Exception-based Directed Acyclic Graph,” i.e., an “EDAG.” An “exception” is a clause that has been put alongside a condition, such that, if the condition fails to be satisfied, this clause, called an exception, shall be assumed to be the conclusion for the decision tree. Gaines, however, also fails to address the issue of variable ordering.

Steven J. Friedman and Kenneth J. Supowit, in their research paper, “Finding the Optimal Variable Ordering for Binary Decision Diagrams,” published in “IEEE Transactions on Computers,” Vol. 39, No. 5 in May 1990, discuss a method for finding the optimal variable ordering where the variables of the decision tree are restricted to having only two values, namely true or false. The representation thus restricts the segmentation of a branch into at most two branches by checking on a given condition to be either true or false. This method is intended to be used in the design of electronic circuits only where the outcome of a “decision” is binary. The method cannot be applied directly to a general decision tree where the variables cannot be restricted to the binary values of either true or false.

Given the limitations of the prior art, there exists a need for a method to create an understandable representation of complex decision trees that is both comprehensible by a human user and is computable in practically feasible amounts of time.

SUMMARY OF THE INVENTION

The invention comprises a method for transforming a large decision tree into a more compact representation to ease use and interaction by a human user. More particularly, the invention comprises a method for transforming an input decision tree into an optimal compact representation by computing a particular ordering of variables in the decision tree that first leads to a Directed Acyclic Graph, or “DAG,” with a minimum number of nodes. The method then converts the DAG into a comprehensible exception-based DAG, or “EDAG,” with exactly one exception. The optimized EDAG presents the same information and knowledge structure provided in the original input decision tree, but in a more compact representation.

The method of the invention includes the following novel features:

-   -   1. An optimal ordering of the variables in the decision tree is         computed, such that when the original input decision tree is         converted to a DAG, the DAG has a minimum number of nodes over         all possible orderings of variables. This DAG structure is         referred to as the “Optimal Model”.     -   2. If all possible variable orderings were evaluated, then         computations on the order of n! (i.e., n factorial, where n         denotes the number of variables) would be required. This method         performs computations on subsets of the sets of variables         instead of the orderings of those variables, requiring far fewer         computations. This method significantly reduces computational         requirements since the number of computations is reduced to the         order of 2^(n), i.e., 2 raised to the power of n, as opposed to         n!.     -   3. An Optimal Model having a minimum number of nodes is created         during the process of computing the optimal ordering.     -   4. The method works for decision trees whose variables may have         a plurality of conditions. The method does not require, for         example, that the variables have only two conditions, e.g., true         or false, as found in Binary Decision Diagrams that support only         two possible states of variables.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical initial input decision tree processed via the method of the invention;

FIG. 2 is an illustration of an expansion of the input decision tree shown in FIG. 1 into a collection of decision chains according to a step of the invention;

FIG. 3 is an illustration of the merging of action nodes of the collection of decision chains in FIG. 2 according to a step of the invention;

FIG. 4 is an illustration of the rearrangement of nodes of the illustration in FIG. 3 according to a step of the invention;

FIG. 5 is an illustration of the merger of the rearranged nodes of FIG. 4 to remove identical subtrees, according to a step of the invention;

FIG. 6 is an illustration of a merged model of the nodes of FIG. 5, according to the invention;

FIG. 7 is an illustration of a nodal rearrangement of the nodes in FIG. 6, according to a step of the invention;

FIG. 8 is an illustration of the merger of nodes of the nodal rearrangement of FIG. 7, according to a step of the invention;

FIG. 9 is an illustration of a merged model of the nodal arrangement of FIG. 8, according to a step of the invention;

FIG. 10 is an illustration of an optimal model for the complete set of variables, according to the invention;

FIG. 11 is an illustration of an optimally ordered EDAG, according to the invention; and,

FIG. 12 is a flow diagram of the methodology of the invention.

FIGS. 13A and 13B illustrate, respectively, the input decision tree having 23 nodes of the credit card approval example transformed into an output EDAG having 14 nodes, according to the methodology of the invention;

FIGS. 14A and 14B illustrate, respectively, an input decision tree having 75 nodes transformed into an output EDAG having 18 nodes, according to the methodology of the invention; and,

FIGS. 15A and 15B illustrate, respectively, an input decision tree having 125 nodes transformed into an output EDAG having 23 nodes, according to the methodology of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The method of the invention is described via an illustrative example dealing with a decision tree used in determining whether to issue a credit card to an applicant or determining what type of credit card to issue to an applicant. FIGS. 1 through 11 provide a graphical illustration of the various steps of the method of a preferred embodiment of the invention, based on an exemplary input decision tree according to the illustrative example, using a specific set of variables, conditions, values, and, outcomes. FIG. 12 is a flow diagram illustrating the steps of the preferred embodiment of the invention, as applied to any input decision tree.

The exemplary input decision tree of FIG. 1 illustrates the decision process for determining whether a particular type of credit card should be issued to a person, depending on the value of certain variables. The variables, possible values, and outcomes included in the example decision tree of FIG. 1 include:

-   -   1. Job: service or business;     -   2. Income: <$100K or >$100 k;     -   3. Assets: <$200K or >$200K; <$500 or >$500K     -   4. Outcomes: NoCard, Silver, Gold, Platinum, Bronze

The following detailed description of the invention follows the steps of the method shown in the flow diagram of FIG. 12 while simultaneously referring to application of the method to the example credit card approval case, which is illustrated in FIGS. 1-11.

Although shown in relation to a decision process for determining the type of credit card to issue to an applicant, the present invention is applicable to creating optimal compact representations of other types of tree structures used for other purposes. For example, the present invention can be used to optimize or analyze, among others, dynamic decision trees for predicting outcomes; for determining optimal paths associated with a project plan consisting of various predetermined procedures; for assessing the true and proximate causes of a particular chain of historical events; and, for other knowledge structures which may lend themselves to initial representation in a tree structure.

As shown in FIG. 1, the initial input for the method of the invention is a decision tree. FIGS. 2 through 10 illustrate the various intermediate steps of the method as applied to the example credit card approval case. As shown in FIG. 11, the ultimate optimized output of the method is an EDAG having an optimal order of variables, creating a more comprehensible and compact representation of the original decision tree for presentation to a user. Now, the various steps of the method of a preferred embodiment of the invention are described.

Step 1: Convert the input decision tree to a collection of independent Decision Chains having the same length: This entire Step 1 corresponds to Box 2 in the flowchart of FIG. 12. A decision chain is a path from the root of the tree to a leaf node such that each node, except the root, has a single parent and each node, except a leaf, has a single child. The method of the invention generates decision chains, each of which has one and only one node for each variable found in the original input decision tree. If a particular decision chain does not initially have a node corresponding to, for example, variable A, the method inserts a “dummy” or “true” node corresponding to variable A. If a particular decision chain has more than one node corresponding to, for example, variable B, the method merges the conditions for all of those nodes into a single node for variable B. As a result, this first step of the method creates a collection of decision chains, all of which have the same length. In addition, the first step of the method sorts the variables within the decision chains so that the variables in each decision chain appear in the same order. FIG. 2 illustrates the resulting decision chains for the initial decision tree shown in FIG. 1, created by application of this step of the method.

The method of the invention also “normalizes” all the conditions of each variable level used in the decision chains so that every condition is as simple and as unambiguous as possible. The “normalization” procedure may significantly increase the number of decision chains, which also increases memory requirements and computational complexity. The normalization procedure, preserving mutual exclusivity, is described in more detail below. An additional embodiment of the invention is described later which provides an alternative method to minimize memory requirements and computational complexity.

As shown in FIG. 3, after all the leveled decision chains have been created, the method merges, all of the action or outcome nodes, i.e., the leaves, or leaf nodes, to reduce node duplication and to prepare the nodal arrangement for subsequent merging steps. FIG. 3 illustrates the result of merging the action nodes in the example credit card approval case, creating individual action nodes of NoCard, Bronze, Silver, Gold, and, Platinum.

Steps to “normalize” i.e., preserve mutual exclusivity, conditions for a variable level: The following steps normalize the conditions for a variable level:

-   -   a) Every variable takes values from a certain domain. Using the         variable conditions found in the original decision tree,         partition a variable domain into the smallest possible set of         non-overlapping value sets, i.e., a partitioning schema, so that         every possible domain value belongs to one and only one value         set. For example, the domain of variable Job can be partitioned         into two sets: Service and Business; the domain of variable         Income can be partitioned into two ranges: Income<100K and         Income>100K; the domain of variable Assets can be partitioned         into three ranges: Assets<200K, 200K<Assets<500K and         Assets>500K.     -   b) Examine every condition for each variable level and if a         particular condition can be satisfied by a value from more than         one value set, replace that condition by a collection of         conditions so that there is no value that can satisfy more than         one of those conditions. For example, Assets>200K would be         replaced with 200K<Assets<500K and Assets>500K.     -   c) Replace the decision chain containing the original condition         with a set of decision chains that are identical except for each         having a different condition on the variable in question. The         same procedure is applied to all conditions including “true”         nodes and nodes that were created in order to ensure that any         given variable only occurred once in a decision chain.

Step 2: Construct an ordered power set: This Step 2 corresponds to Box 3 in the flowchart of FIG. 12. After converting the original input decision tree to a collection of independent decision chains of the same length, the method constructs an ordered power set. A power set, denoted by “S,” is a set of all possible subsets of the variables in the original decision tree. The method arranges the elements of S in ascending order of cardinality, i.e., according to the number of elements in the element set, to create an ordered power set. For example, the ordered power set for the set of variables in the example decision tree of FIG. 1 is:

-   -   {         -   {Job}, {Income}, {Assets},         -   {Job, Income}, {Job, Assets}, {Income, Assets},         -   {Job, Income, Assets}         -   }

Step 3: Compute an Optimal Model for each element from S having cardinality of one: This Step 3 is shown as Boxes 6 and 7 in the flowchart of FIG. 12. After constructing an ordered power set, S, the method selects a first element C from S having a cardinality value of one and computes the Optimal Model for that element. Elements in the illustrative example having a cardinality of one include {Job}, {Income}, and {Assets}. Since the cardinality of the selected first element, C, is one, the element contains only one variable v and the Optimal Model is developed according to the following steps:

-   -   3.1 Create a copy of the decision chain set structure and         manipulate it by moving the nodes of the variable v to the         bottom level of the decision chain set structure, just above the         leaves, i.e., the action or outcome nodes.

FIG. 4 illustrates this step as applied to the example case, where v=Income and where the nodes of the level corresponding to, Income are moved below the level corresponding to Assets, to just above the action leaves, which determine the type of Card issued to an applicant.

-   -   3.2 Merge the nodes at the leaf level and the level with         variable v to obtain the minimum number of nodes on those two         levels. Refer to the section below, Description of Merging         Procedures, for detailed merging steps.

FIGS. 5 and 6 illustrate these steps as applied to the example credit card approval case. As shown in the transformations from FIG. 4 to FIG. 5 and then FIG. 6, the node count and number of decision chains continues to be greatly reduced by the application of the method of the invention.

This newly created structure is the Optimal Model for element C, having a cardinality of one, which is placed into memory storage for further use. The method then computes the Optimal Model for all other elements of cardinality one, and stores each result in memory. For the example case, optimal models for {Income}, {Job}, and {Assets} are each placed into memory storage.

Description of the Merging Procedures

Redundant nodes of variable v are merged to keep the resulting graphic structure as compact as possible, according to two primary methods:

-   -   1. Each node with variable v sharing the same child and the same         condition is merged to a single node with that same condition         and pointed to by all of the parents of the nodes being merged.         FIG. 5 illustrates this merging method for nodes of variable         “Income,” based on the example case.     -   2. Each node of the variable v having the same children and the         same ancestors, not just immediate parents, in the decision         chains is merged. The nodes of v are replaced with a single node         with a complex condition that is a union of conditions from the         nodes being merged. The condition of the merged node has to         evaluate to “true” for all the values that evaluate to “true”         for the nodes being replaced. If the new, complex, condition of         the merged node covers the whole domain, then it is a “true”         node. True nodes for variable v can be deleted and their parents         shall instead point to the children of the true node. FIG. 8         illustrates this merging method where true nodes of the variable         “Job” are removed.

Merging also takes place in the decision chains above nodes of variable v. Multiple occurrences of exactly identical decision chains, i.e., the upper, still un-merged parts, are removed; and outgoing arcs are drawn to the all of the children of removed parts. FIG. 6 illustrates this process for the example credit card approval process case.

Step 4: Compute an Optimal Model for all element sets with cardinality greater than one: This Step 4 is shown as Boxes 9 to 17 in the flowchart of FIG. 12. After computing an Optimal Model for each element of cardinality one, the method proceeds to compute an Optimal Model for all element sets having cardinality greater than one. In the illustrative example, there are element sets having cardinality values of two and three. First, an element C having cardinality greater than one is selected from S in the order of cardinality and the Optimal Model is computed for that element. Since the cardinality of C is greater than one, the element contains more than one variable, and the Optimal Model is determined as follows:

-   -   Step 4.1: Choose a variable v from the subset element C. Let all         the remaining variables in C except v be called the         complimentary set of v, denoted by C′ (v). For the example case,         where the subset element C having a cardinality of two is {Job,         Income}, v is Job and C′ (v)={Income}     -   Step 4.2: Retrieve the Optimal Model for the complementary set         C′ (v) from memory storage and make a copy of it. Since element         C is chosen in order of cardinality, the method ensures that the         optimal structure for C′ (v) has already been calculated. For         the example case, the optimal model copied from memory storage         is for C′ (v)={Income}.     -   Step 4.3: Move the nodes for variable v (Job) in the copied         model downward in each decision chain until all of the nodes         corresponding to the remaining variables in C′ (v) {Income} are         below nodes for variable v (Job) and all of the nodes for         variables not in C {Assets} are above the nodes for variable v         (Job). FIG. 7 illustrates this step of the method of the         invention as applied to the example case.     -   Step 4.4: Simplify the nodal structure to create a merged model         by merging redundant nodes of variable v according to the         previously described merging procedures, in the section,         Description of Merging Procedures, described above.     -   Step 4.5: For the merged model created in step 4.4, compute the         number of nodes on levels corresponding to variables from subset         element C. Check memory storage for an available model for         subset element C. If an available model does not exist in memory         storage, place the merged model created in Step 4.4 in memory         storage. If a stored model does exist, determine if the stored         model has more nodes than the current merged model on the levels         corresponding to variables from subset element C. If the stored         model has more nodes, replace the existing model in memory         storage with the current merged model.

FIGS. 7, 8 and 9 show application of the above substeps to the example credit card case for a subset element C={Job, Income} where the chosen variable v is Job.

Step 5: Select Optimal Model and restore single root node: Step 5 is shown as Box 18 in the flowchart of FIG. 12. In the process of computing the optimal order for all the elements of the ordered power set, S, a set containing all the variables of the original tree is eventually chosen. The stored model for that set is the overall Optimal Model. The overall Optimal Model is selected and the multiple root nodes are merged to a single root node. FIG. 10 illustrates the Optimal Model for all variables, based on the example credit card case.

Step 6: Select the Global Exception and create the EDAG: This Step 6 is shown as Box 19 in the flowchart of FIG. 12. From the Optimal Model for the entire set of variables selected in Step 5, select the leaf node having the maximum number of parent nodes, and declare that node to be the global exception. Remove all nodes that are parents of the global exception node from the Optimal Model along with all edges connected to those nodes. Place the global exception node at the root of the overall tree structure. FIG. 11 illustrates the application of Step 6 to the example credit card approval case where the global exception node has been computed as the NoCard node. In this particular example, all leaf nodes happen to have the same number of parents and so a leaf node is selected arbitrarily.

Consequently, the original input decision tree has been converted to an optimal, compact representation in the form of an optimized and minimized exception-based directed acyclic graph where the entire knowledge structure of the original input decision tree has been retained. Having thus developed an optimized, compact representation, a user can more easily comprehend and interact with the knowledge structure.

Low Memory Method: The method described in Steps 1 though 6 uses a decision chain set structure such that the merged variables are represented as a DAG and un-merged variable levels are represented as decision chains. Because of the normalization procedure described earlier, decision chains can have many redundant nodes and edges, leading to high memory requirements. Consequently, in an additional embodiment of the invention, we describe a low memory method that represents un-merged variable levels as a DAG instead of decision chains. This method is similar to the method described above in Steps 1 through 6, except for the following changes:

In Step 1, the input decision tree is converted to a directed acyclic graph, i.e., a “DAG,” as follows:

-   -   a) A collection of leveled, read-once decision chains is created         and normalized, as described for Step 1 of the original method.     -   b) Starting from the leaf nodes and moving toward the root, the         nodes are merged level by level according to the methods         described above in the section, Description of Merging         Procedures. This results in a leveled, read-once DAG.

The DAG structure reduces memory consumption, but adds the additional requirement of performing “local” normalization and simplification steps during each level manipulation. In step 3, when moving nodes of variable v to just above leaf nodes and in step 4, when moving nodes of variable v to just above the nodes of the variables in C′ (v), the movement has to take place one level at a time, rather than movement directly across multiple levels, as is done in the first embodiment of the invention described above. Local normalization and simplification proceed according to the following steps:

-   -   a) The nodes of variable v are interchanged with the child         level, c. The child level, c, is the level just below the level         of the nodes of variable v. Copies of the nodes of variable v         and child nodes c are made such that there is a one-to-one         relation between all nodes of the variables v and c. Then, the         nodes of variable v are interchanged with the child nodes c.     -   b) The mutual exclusivity for variable level, c, is maintained,         as described above, in the section, “Steps to “normalize” i.e.,         preserve mutual exclusivity, conditions for a variable level”.     -   c) The nodes of level v and c are merged according to the         methods described above in the section, Description of Merging         Procedures. For all the nodes of variables v and above, during         merging methods, refer to only the immediate parent node instead         of the entire decision chain above that node.     -   d) Repeat steps a) to c) until all the nodes of variable v are         pointing to only nodes of variables in C′(v).

When applied in practice, the invention significantly simplifies the graphic structure displayed to a user. For example, FIGS. 13A and 13B illustrate the input decision tree and output EDAG of the credit card approval example. The input decision tree in FIG. 13A contained 23 nodes; the corresponding output EDAG in FIG. 13B contained only 14 nodes, representing an approximate 39% reduction in size and complexity.

As the size and complexity of the input decision tree increases, the impact of the application of the invention is even more dramatic. For example, FIGS. 14A and 14B illustrate another example of the ability of the present invention to transform a large complex, decision tree into a compact representation of the same knowledge. In this case, the input decision tree in FIG. 14A contained 75 nodes; the corresponding output EDAG in FIG. 14B contained only 18 nodes, representing an approximate 76% reduction in size and complexity.

FIGS. 15A and 15B illustrate another example of the ability of the present invention to transform a large complex, decision tree into a compact representation of the same knowledge. In this case, the input decision tree in FIG. 15A contained 125 nodes; the corresponding output EDAG in FIG. 15B contained only 23 nodes, representing an approximate 82% reduction in size and complexity.

As can be seen by the above description and presentation of the tangible results associated with the invention, the invention provides a useful and computationally tractable means by which complex and confusing decision trees can be transformed into a smaller, simpler, and more comprehensible format, which will ease understanding and utility of the knowledge represented by the initial input decision tree.

Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below. 

1. A computer implemented method for transforming a decision tree having a plurality of variables, a root node, intermediate nodes, and leaf nodes into an optimized compact graphical representation for display to a user, comprising: inputting the decision tree; converting the leveled, read-once decision tree into a collection of independent decision chains, each of the independent decision chains having one level for each variable and wherein the variables are arranged in the same order, thereby creating a decision chain set structure; normalizing the decision chain structure and merging the leaf nodes; constructing an ordered power set from the plurality of variables to form elements comprising one or more variables; computing an optimal model graph for each element of the ordered power set having cardinality of one; computing an optimal model graph for each element of the power set having cardinality greater than one; selecting an optimal model graph for an element of the power set containing all variables; transforming the selected optimal model graph to an optimal model graph having a single root node; selecting a global exception from the optimal model graph having a single root node; and, creating an exception-based directed acyclic graph from the optimal model having a single root node graph wherein the exception-based directed acyclic graph is comprised of the least number of nodes and the root node is associated with the global exception, thereby creating an optimized and compact representation of the original input decision tree for display to a user; wherein computing an optimal model graph for all element sets having a cardinality greater than one further comprises: selecting and removing a variable from each element to form a complimentary corresponding subset comprised of remaining variables of the element; retrieving a previously computed optimal model graph for the complementary subset from storage and making a copy of the optimal model graph of the complementary subset; moving all nodes for the selected variable in the copied optimal model graph downward in each decision chain until all nodes corresponding to remaining variables in the complementary subset are below nodes for the selected variable and all nodes for variables not in the element are above nodes for the selected variable; simplifying a nodal structure of the of the copied optimal model graph by merging redundant nodes of the selected variable to create a merged model; computing the number of nodes on levels corresponding to variables from the element; checking storage for an available model for the element; if an available model does not exist in storage, placing the merged model in memory storage; if a stored model does exist, determining if the stored model has more nodes than the current model on the levels corresponding to variables from the element; if the stored model has more nodes, replacing the existing model in storage with the current model.
 2. The method of claim 1, wherein the computing an optimal model graph for each element having a cardinality of one, further comprises: creating a copy of the decision chain set structure; manipulating the decision chain set structure by moving all nodes of a selected variable to a lower level of the decision chain set structure, just above a level of the leaf nodes; and, merging the selected variable nodes to minimize the number of nodes; and, repeating creating, manipulating, and merging for the remaining elements having a cardinality of one.
 3. The method of claim 1 wherein the simplifying the nodal structure by merging redundant nodes of the selected variable to create a merged model further comprises: identifying nodes of the selected variables having identical conditions and identical child nodes; and merging nodes of the selected variables having identical conditions and identical child nodes to a single node; identifying nodes of the selected variables having same ancestors and identical child nodes; replacing nodes of the selected variable having same ancestors and identical child nodes with a single node having a complex condition comprising a union of conditions from the selected variable nodes being replaced; identifying identical decision chains above the selected variable; selecting just one decision chain out of multiple identical chains and removing the additional identical decision chains; and pointing the child nodes of the removed decision chains by the one selected decision chain.
 4. The method of claim 3 where the complex condition of the merged node covers the entire domain of the variable and evaluates to “true” for all values that evaluate to “true” for the nodes being merged, further comprising deleting true nodes for the variable such that parent nodes of the true node point to the corresponding children of that true node.
 5. The method of claim 1 adapted for low memory requirements and low computational complexity, further comprising: converting the input decision tree to a decision chain structure by replacing any variables missing in any path of the tree with true nodes and replacing repeated occurrences of variables along any path with single nodes; and, normalizing the decision chain structure and merging any identical chains starting from the leaf nodes and merging level by level for all identical chains such that the entire decision chain structure is merged to a directed acyclic graph; moving all nodes of a selected variable to a lower level one level at a time, rather than movement directly across multiple levels.
 6. The method of claim 5 wherein the moving all nodes of a selected variable to a lower level one level at a time, further comprises: copying and duplicating selected variable nodes and corresponding child nodes such that there is a one-to-one relation between all nodes of the selected variable and all corresponding child nodes; interchanging the selected variable nodes with the child nodes; maintaining mutual exclusivity for the child nodes; merging nodes at the selected variable level and corresponding child level but referring only to immediate parent nodes instead of the entire decision chain above that node; and, repeating copying and duplicating, interchanging, maintaining, and merging until all selected variable nodes are pointing only to nodes of remaining variables in the complementary subset.
 7. A computer implemented method for transforming a knowledge structure organized as a decision tree having a plurality of variables into a compact exception-based directed acyclic graph for display to a user, comprising: transforming the decision tree into a collection of leveled read-once decision chains; transforming the leveled read-once decision chains into a directed acyclic graph wherein the directed acyclic graph has the least number of nodes; and, transforming the directed acyclic graph with the least number of nodes into an exception-based directed acyclic graph, providing the simplest graphical representation of the decision logic embodied in the tree for display to a user; wherein transforming the decision tree, transforming the leveled read-once decision chains into a directed acyclic graph and transforming the directed acyclic graph with the last number of nodes into an exception-based directed acyclic graph further comprises: moving each variable of the directed acyclic graph in turn to the bottom of the directed acyclic graph and memorizing the corresponding directed acyclic graph for each variable; for each of the corresponding directed acyclic graphs, moving another variable not at the bottom level to a level immediately above the bottom level; from a set of directed acyclic graphs having a specific subset of variables, selecting and memorizing the directed acyclic graph having the least number of nodes for those variables; repeating the transforming until a directed acyclic graph having the least number of nodes is selected and memorized; converting the directed acyclic graph having the least number of nodes to an exception-based directed acyclic graph; selecting a conclusion of the exception-based directed acyclic graph having a greatest number of incoming edges and removing the conclusion from the exception-based directed acyclic graph; removing all nodes leading exclusively to the removed conclusion; removing all edges leading to the removed conclusion or the removed nodes; and, establishing the removed conclusion as a global exception of the exception-based directed acyclic graph; thereby creating a compact graphic representation for presentation to a user.
 8. A computer implemented method for transforming an input decision tree having a plurality of variables, a root node, intermediate nodes, and leaf nodes into an optimal exception-based directed acyclic graph for output and display to a user, comprising: inputting a decision tree; converting the decision tree to a collection of leveled, read-once decision chains, creating a decision chain structure; normalizing the decision chain structure and merging the leaf nodes; creating a power set of the plurality of variables consisting of elements comprised of one or more variables; ordering the elements of the power set according to increasing cardinality of the elements; choosing an element from the ordered power set having a cardinality of 1; choosing a variable from the chosen element; copying the decision chains structure and moving the nodes of the variable selected in step g to the level immediately above the leaf nodes; merging the nodes for a level corresponding to the selected variable and then merging the leaf nodes level to obtain a merged structure having a minimum number of nodes for those two levels; storing the merged structure into memory storage; determining whether all of the elements from the ordered power set having cardinality 1 have been considered; if all elements of the power set having cardinality 1 have not been considered, returning ordering the elements of the power set and choosing a next element in the power set having cardinality 1; if all elements of the power set having cardinality 1 have been considered, determining whether all of the elements from the ordered power set having cardinality greater than 1 have been considered; if all elements of the ordered power set having cardinality greater than 1 have not been considered, choosing a first element of the remaining elements of the power set according to order of cardinality; choosing a first variable from the first element and designating the set of remaining variables as a second element; retrieving from memory storage an optimal directed acyclic graph for the second element and copying the optimal directed acyclic graph to working memory; manipulating the copied optimal directed acyclic graph by moving nodes for the chosen first variable down such that all variables in the second element are below the nodes for the first variable and all of the nodes for those variables not in the first element are above the nodes for the first variable; merging the nodes at the level of the first variable to obtain a minimum number of nodes for the first variable and to create a merged directed acyclic graph; determining if there exists a directed acyclic graph in memory storage for the first element and if not, placing the merged directed acyclic graph into memory storage; determining if there exists a directed acyclic graph in memory storage for the first element, and if so, comparing the merged directed acyclic graph with the stored directed acyclic graph; where the merged directed acyclic graph is comprised of a lesser number of nodes than the stored directed acyclic graph, storing the merged directed acyclic graph in memory storage while erasing the previously stored directed acyclic graph; determining whether all variables in the first element have been considered; if all variables in the first element have not been considered, repeating from the choosing of the first variable for the next variable in the first element; where all variables in the first element have been considered, establishing the last directed acyclic graph stored for the first element as the optimal directed acyclic graph for the first element; determining whether all elements of the optimally ordered power set have been considered; if all elements of the power set have not been considered, returning to determining whether all of the elements from the ordered power set having cardinality greater than 1 have been considered and choosing a next element in the power set according to order of cardinality; if all elements of the power set have been considered, establishing the directed acyclic graph for the element with all variables as the optimal directed acyclic graph for the input decision tree; converting the optimal directed acyclic graph to a single root graph; designating a conclusion node of the single root graph having the maximum number of parents nodes as a global exception; converting the single root graph with the global exception to create an optimal exception-based directed acyclic graph; and outputting the optimal exception-based directed acyclic graph to a display for presentation to a user.
 9. An article comprising a tangibly embodied machine-readable storage medium operable to cause one or more machines to result in operations comprising: inputting a decision tree having a plurality of variables, a root node, intermediate nodes and leaf nodes wherein each node is associated with at least one or more variables; converting the decision tree into a collection of leveled, read-once independent decision chains having an equal number of levels to create a decision chain set structure; normalizing the decision chain structure and merging the leaf nodes; constructing an ordered power set from the plurality of variables to form elements comprising one or more variables; computing an optimal model graph for each element of the ordered power set having cardinality of one; computing an optimal model graph for each element of the power set having cardinality greater than one; selecting an optimal model graph for an element containing all variables having the least number of nodes; transforming the selected optimal model graph to one with a single root node; selecting a global exception from the optimal model graph having a single root node; and, creating an exception-based directed acyclic graph from the optimal model single root node graph wherein the exception-based directed acyclic graph is comprised of the least number of nodes and the root node is associated with a global exception, thereby creating an optimized and compact representation of the original input decision tree for display to a user; wherein the operation of computing an optimal model for all element sets having a cardinality greater than one further comprises: selecting and removing a variable from each element to form a complimentary corresponding subset comprised of remaining variables of the element; retrieving a previously computed optimal model graph for the complementary subset from storage and making a copy of the optimal model graph of the complementary subset; moving all nodes for the selected variable in the copied optimal model graph downward in each decision chain until all nodes corresponding to variables in the complementary subset are below nodes for the selected variable and all nodes for variables not in the element are above nodes for the selected variable: simplifying a nodal structure of the of the copied optimal model graph by merging redundant nodes of the selected variable to create a merged model; computing the number of nodes on levels corresponding to variables from the element; checking storage for an available model for the element; if an available model does not exist in storage, placing the merged model in memory storage; if a stored model does exist, determining if the stored model has more nodes than the current model on the levels corresponding to variables from the element; if the stored model has more nodes, replacing the existing model in storage with the current model.
 10. The article of claim 9, wherein the operation of computing an optimal model graph for each element having a cardinality of one, further comprises: creating a copy of the decision chain set structure; manipulating the decision chain set structure by moving all nodes of a selected variable to a lower level of the decision chain, just above a level of the leaf nodes; and, merging the selected variable nodes to minimize the number of nodes.
 11. The article of claim 9 wherein the operation of simplifying the nodal structure by merging redundant nodes of the variable to create a merged model further comprises: identifying nodes of the selected variables having identical conditions and identical child nodes; and merging nodes of the selected variables having identical conditions and identical child nodes to a single node; identifying nodes of the selected variables having the same ancestors and identical child nodes; replacing nodes of the selected variable having the same ancestors and identical child nodes with a single node having a complex condition comprising a union of conditions from the selected variable nodes being replaced; identifying identical decision chains above the selected variable; selecting just one decision chain out of multiple identical chains and removing the additional identical decision chains; and pointing the child nodes of the removed decision chains by the one selected decision chain.
 12. The article of claim 11 where the complex condition of the merged node covers the entire domain of the variable and evaluates to “true” for all values that evaluate to “true” for the nodes being merged, further comprising the operation of deleting true nodes for the variable such that parent nodes of the true node point to the corresponding child nodes of the true node.
 13. The article of claim 9 adapted for low memory requirements and low computational complexity, further comprising: converting the input decision tree to a decision chain structure by replacing any variables missing in any path of the tree with true nodes and replacing repeated occurrences of variables along any path with single nodes; and, normalizing the decision chain structure and merging any identical chains starting from the leaf nodes and merging level by level for all identical chains such that the entire decision chain structure is merged to a directed acyclic graph; moving all nodes of a selected variable to a lower level one level at a time, rather than movement directly across multiple levels.
 14. The article of claim 13 wherein moving all nodes of a selected variable to a lower level one level at a time, further comprises: copying and duplicating selected variable nodes and corresponding child nodes such that there is a one-to-one relation between all nodes of the selected variable and all corresponding child•nodes; interchanging the selected variable nodes with the child nodes; maintaining mutual exclusivity for the child nodes; merging nodes at the selected variable level and corresponding child level but referring only to immediate parent nodes instead of the entire decision chain above that node; and, repeating copying, interchanging, maintaining, a and merging, until all selected variable nodes are pointing only to nodes of remaining variables in the complementary subset.
 15. An article comprising a tangibly embodied machine-readable storage medium operable to cause one or more machines to result in operations comprising: inputting a decision tree; converting the decision tree to a collection of leveled, read-once decision chains, creating a decision chains structure; normalizing the decision chain structure and merging the leaf nodes; creating a power set of the plurality of variables consisting of elements comprised of one or more variables; ordering the elements of the power set according to increasing cardinality of the elements; choosing an element from the ordered power set having a cardinality of 1; choosing a variable from the chosen element; copying the decision chains structure computed in step b and moving the nodes of the variable selected in step g to the level immediately above the leaf nodes; merging the nodes for a level corresponding to the chosen variable and then merging the leaf nodes level to obtain a merged structure having a minimum number of nodes for those two levels; storing the merged structure computed in step i into memory storage; determining whether all of the elements from the ordered power set having cardinality 1 have been considered; if all elements of the power set having cardinality 1 have not been considered, returning to choosing an element from the ordered power set having a cardinality of 1 and choosing a next element in the power set having cardinality 1; if all elements of the power set having cardinality 1 have been considered, determining whether all of the elements from the ordered power set having cardinality greater than 1 have been considered; if all elements of the ordered power set having cardinality greater than 1 have not been considered, choosing a first element of the remaining elements of the power set according to order of cardinality; choosing a first variable from the first element and designating the set of remaining variables as a second element; retrieving from memory storage an optimal directed acyclic graph for the second element and copying the optimal directed acyclic graph to working memory; manipulating the copied optimal directed acyclic graph by moving nodes for the chosen first variable down such that all variables in the second element are below the nodes for the first variable and all of the nodes for those variables not in the first element are above the nodes for the first variable; merging the nodes at the level of the first variable to obtain a minimum number of nodes for the first variable and to create a merged directed acyclic graph; determining if there exists a directed acyclic graph in memory storage for the first element and if not, placing the merged directed acyclic graph into memory storage; determining if there exists a directed acyclic graph in memory storage for the first element, and if so, comparing the merged directed acyclic graph with the stored directed acyclic graph; where the merged directed acyclic graph is comprised of a lesser number of nodes than the stored directed acyclic graph, storing the merged directed acyclic graph in memory storage while erasing the previously stored directed acyclic graph; determining whether all variables in the first element have been considered; if all variables in the first element have not been considered, repeating from choosing the first variable from the first element for the next variable in the first element; where all variables in the first element have been considered, establishing the last directed acyclic graph stored for the first element as the optimal directed acyclic graph for the first element; determining whether all elements of the optimally ordered power set have been considered; if all elements of the power set have not been considered, returning to determining whether all of the elements from the ordered power set having cardinality greater than 1 have been considered and choosing a next element in the power set according to order of cardinality; if all elements of the power set have been considered, establishing the directed acyclic graph for the element with all variables as the optimal directed acyclic graph for the input decision tree; converting the optimal directed acyclic graph to a single root graph; designating a conclusion node of the single root graph having the maximum number of parents nodes as a global exception; converting the single root graph with the global exception to create an optimal exception-based directed acyclic graph; and outputting the optimal exception-based directed acyclic graph to a display for presentation to a user. 